(i) Technical Field
The present invention relates to shutter blade speed adjustable mechanisms that change the amount of drive force of a biasing member for driving a shutter blade to adjust the running speed of the shutter blade, and focal plane shutters that have the same.
(ii) Related Art
There is conventionally known a focal plane shutter which is employed in an optical device. The focal plane shutter uses a ratchet member and an engagement member, which has a pawl portion engaging a ratchet teeth of the ratchet member, in order to adjust the running speed of leading and trailing blades (for example, see Japanese Unexamined Patent Application Publication No. 2003-66508). Shutter blades, which configure the leading and trailing blades, are pivotally supported by two arms pivotally attached in a board having an opening so as to be aligned in lengthwise directions of two arms. The leading and trailing blades are overlapped and expanded by the rotation of a drive lever (opening or closing lever) which engages the arm and is pivotally supported by the board.
Further, each of the leading and trailing blades is provided with a drive spring serving as a biasing member. The biasing force of the drive spring is used as a drive source for the shutter blade. Here, there is provided the ratchet member having the ratchet teeth and the engagement member holding the ratchet member in a given position to adjust the running speed by changing the biasing force of the drive spring.
The adjustment of the running speed of the shutter blade in the focal plan shutter is generally configured as follows. The drive spring serving as the biasing member is provided between the drive lever and the ratchet member. The ratchet member is held by engaging the pawl portion of the engagement member with the ratchet teeth. Here, changing the engagement relationship between the pawl portion of the engagement member and the ratchet teeth, beforehand, causes the ratchet member to change its stop position, thereby changing the attachment state of the biasing member. Accordingly, the biasing force of the drive spring is changeable. That is, by adjusting the engagement relationship between the pawl portion and the ratchet teeth before the operation of the shutter, the running speed of the shutter blade is adjustable. Further, the pawl portion of the engagement member is composed of plural pawls, which are engaged with plural ratchet members different in phase from each other, thereby controlling the running speed of the shutter blade by finely arranged steps.
Additionally, there has been implemented a shutter blade speed adjustable mechanism which gradually adjusts the biasing force of the spring with a worm mechanism to gradually control the running speed of a shutter blade.
In such a blade speed adjustable mechanism, the plural pawl portions of the engagement member, which engages the ratchet teeth to hold the ratchet member, are similar to each other in length so as to engage the plural ratchet teeth, which are different in phase from each other.
However, in order to increase the number of the steps of speed adjustment in such conventional configurations, it is conceivable to reduce a pitch distance of the ratchet teeth without increasing the number of the pawl portions of the engagement portion. Alternately, it is conceivable to increase the number of the pawl portions of the engagement portion and the types of the ratchet portions, which are different in phase from each other, to respectively engage the pawl portions with the ratchet teeth.
As a result, when the pitch is made small without increasing the number of the pawl portions of the engagement member, grooves, which are capable of engaging the pawl portion, of the ratchet portion are made shallow. This arises a problem that the ratchet member is not held with certainty. Further, when the number of the pawl portions of the engagement member is increased and the pawl portions respectively engage the ratchet teeth different in phase from each other, the thickness of the ratchet teeth is increased in its rotational axis direction, so that large space is needed for the increase.
Additionally, a blade speed adjustment mechanism using a worm mechanism has a problem of reducing its size and cost, in comparison to the blade speed adjustment mechanism using the ratchet member.